Radiant tubular heater and method of heating



June 26, 1956 K. PERMANN 2,751,893-

RADIANT TUBULAR HEATER AND METHOD OF HEATING Filed July 21, 1952 5 Sheets-Sheet l K P m 5H WEI/www June 26. 1956 Filed July 21, 1952 K. PERMANN RADIANT TUBULAR HEATER AND METHOD OF HEATING 5 Sheets-Sheet 2 Fg Z lnvenJror:

Karl Pes-mann E |.3

his A? ornzg June 26. 1956 K PERMANN 2,751,893

RADIANT TUBULAR HEATER AND METHOD OF HEATING Filed July 21, 1952 5 Shea-:ts-Shee*v 3 FqA lnvznor v Karl Permann K. PERMANN 5 Sheets-Sheet 4 RADIANT TUBULAR HEATER AND METHOD OF HEATING -,@WEW

June 26, 1956 Filed July 21, 1952 5 Sheets-Sheet 5 K.. PERMANN RADIANT TUBULAR HEATER AND METHOD OF HEATING f f/,f/

June 26, 1956 Filed July 21, 1952 -points circumferentially about the tube.

United States Patent RADIANT TUBULAR HEATER AND METHOD F HEATING Karl Permanu, Oakland, Calif., assigner to Shell Development Company, Emeryville, Calif., a corporation of Delaware Application July 21, 1952, Serial No. 299,987

26 Claims. (Cl. 122-275) This invention relates to the art of heating tubes or pipes byvmeans of radiant heat. In one aspect, the invention relates to a method of adsorbing radiant heat to effect a distribution of heat circumferentially about the tubes, and to an arrangement of radiating surfaces to eltect such a distribution. In a more specific application, it relates, further, to a method of heating tubes uniformly or in any desired gradient along their lengths with radiant heat, and to a red, tubular heater wherein Vthis method is carried out. Further, the invention is concerned with a radiant tubular converter provided with a pre-heating section which is thermally controllable independently of the radiant section for supplying process uids at any desired temperature to the radiant section.

The heating method and heating arrangement are particularly suited for application in heaters which are used either for catalytic or non-catalytic reactions, usually endothermic, where the temperature of the reacting medium, herein referred to as the process fluid, is maintained for the necessary reaction time at an elevated temperature which is near the decomposition temperature, and i wherein it is important to maintain accurately a steady temperature of such stream. In such situations even a slight temperature drop may result in deposition of heavy residue and plugging of the tubes, while a rise in temperature above the proper level leads to decomposition and other undesired reactions. A specific example is the conversion of ethylene dichloride to form vinyl chloride, which is an endothermic reaction taking place at about 960 F. The invention is not, however, limited to such specialized uses and the method and apparatus may be used to advantage wherever a uid is to be maintained at an even or readily regulated temperature by radiant heat.

It is an object of the invention to provide a tubular i heater and a method of heating tubes wherein radiant heat is used to greater advantage to supply heat to various Specifically, -v it is an object to position the tube in relation to a source 'of radiant heat in a manner to avoid direct impingement on the tube of the most intense part of the initial radiation and to distribute the radiant heat from the same source to opposite sides of the tubes by re-radiation from opposed radiating walls.

It is a further object to provide a tubular heater and a method of heating tubes wherein the absorption of heat is suitably distributed along the length of the tubes, e. g.,

.is uniformly distributed or is distributed along the length of the tubes in accordance with any desired gradient. Ancillary thereto, it is an object to provide a heater and a method of heating wherein the absorption of heat is distributed more evenly around the full circumference of each tube, and among the several tubes when, as is usual, a plurality of tubes are provided, than in known heaters and methods employing radiant heaters.

A further object is to provide an improved tubular heater wherein more even and therefore lower temperatures are maintained in the entire radiant section and, particularly, in the outer wall structure, than in prior radiation type heaters of like heat absorption capacities, whereby maintenance is reduced to a Further objects are to provide a tubular heater of the type described having a preheating section, wherein heat in the efuent gas from the radiant section is used to bring the process iluids to the reaction or conversion temperature; to provide for the facile regulation of the preheating temperature independently of the equilibrium temperature or the rate of heat emission in the radiant section; and to enclose the preheating section by the radiant section, thereby obviating the need for transfer ducts or extension of the radiant section into a preheating section and the insulation which is usually provided for the latter.

Still further objects are to provide a fired tubular heater of the radiant type which is compact in design, requiring but little oor space.

Still further objects of the invention will become apparent from the following description, taken together with the drawings forming a part of this specification and illustrating certain specific embodiments of the invention and showing the best mode contemplated for applying the same, wherein:

Fig. 1 is a typical sectional view of dimensional relation between the beam of radiations and tubes;

Fig. 2 is a vertical sectional View of a heater constructed according to the invention, certain parts being shown in elevation and the conduits for supplying fuel to the burners being omitted;

Fig. 3 is a horizontal sectional view, taken on line 3 3 of Fig. 2;

Fig. 4 is an enlarged horizontal sectional view showing details of one of the radiant burners;

Fig. 5 is a schematic elevation view of the heater showing the fuel supply system;

Fig. 6 is a vertical sectional view of a modified embodiment taken on line 6 6 of Fig. 7; and v Fig. 7 is a horizontal sectional view, taken on line 7-7 of Fig. 6.

According to the present invention the process stream to be heated or to be maintained at an elevated temperature is flowed through one or more radiantly heated tubes located between rst and second radiant walls, and heat is radiated from a source located near, e. g., at, inside or close to, the first wall essentially as a confined beam (as described more particularly hereinafter) against a limited area of the facing surface of the second wall, the tube being situated outside the main part of the confined beam to avoid radiation of the highest intensity from fallingdirectly on the tube. In other words, only' low intensity radiant heat, at the fringe of the conned beam, is radiated directly from the source to the tubes, and the amount of this heat is only a small part, less than 25% and, usually, less than about 18% of the total radiant heat that ultimately falls on the tube. Hence the heat radiated at the source is predominantly immitted on the said facing surface of the second radiant wall. The said sourceof heat occupies only a minor part of the area of the rst radiant wall, leaving a major part of the radiant wall surface exposed for re-radiation of heat, as described later.

The heat immitted on the facing surface of the second radiant wall is partly absorbed and mainly re-radiated from the same surface thereof; a portion of this first reradiation is absorbed by the side of the tube exposed toward the second wall and the remaining part is largely immitted on the facing surface of the rst radiant wall, near which the initial radiation took place. The latter immitted heat is partly absorbed and mainly re-radiated 3 from the same surface of the rst wall, a portion of this second re-radiation being absorbed by the side of the tube exposed to theiirst wall and the remaining part being largely imrnitted on the second wall. Here further partial absorption and re-radiation occur, this etect being repeated untilV the intensity of repeated re-radiations is diminished to a point at which re-radiations can be neglected from further consideration. In this manner the tube, which may be regarded as a narrow heat acceptor, is protected from excessive, onesided overheating, while both sides of the tubei. e., the side facing the first wall and the side facing the second wall, are both heated by re-radiated heat from the same original source of radiant heat. Since the heat is re-radiated successively from the ysaid radiant walls at different angles and from extended areas thereof, far greater than the area of the initial source of radiant heat, the surface of the tube is exposed Ito radiations from many directions emanating over large radiating area, which results in a more even heating of the rtubes about the circimiferences thereof, provided, of course, that the Vtube is spaced apart from any obstruction, such as another tube, by a sufcient distance to avoid jshieiding. VSince the radiant walls radiate energy in all directions, energy from each wall will fall on more than half of the surface of the tube to an extent dependent upon the intervals between tubes, e. g., in excess of three-fourths of the surface of the tube; hence the sides of the tubes mentioned in this specification as exposed to particular walls are not exclusive but overlap.

Although in the foregoing paragraphs only a single primary source of heat at the first wall was mentioned, it should be understood that the invention includes the case wherein a plurality of primary sources of heat are used; such sources of heat may be situated near both radiant wail-S. Thus, heat'may be simultaneously and additionally'radiated from a source near to the second wait and occupying a minor part of the area thereof against a limited area of the first wall essentially as a Vconfined beam without impingement of the main part of the beam on the tube, and this heat may then be similarly Arefradiatedv repeatedly.

In this specification and the claims thereof I use the .termA fimmitted heat to denote the. total radiant heat falling on any surface, including the portions thereof which are absorbed or re-radiated.

This outlined Combination of the principles of divergence of radiating energy is graphically illustrated in Fig. l. The, first radiant wall A, of which a portion is shown, contains radiant type gas or oil burners B having ceramic refractory burner blocks, placed at certain distances a from each other. According to the shape of the burner cavity in the block, the radiant energy which is produced on its incandescent surface is reected (directly radiated). toward and predominantly onto a limited area of the opposite radiant wall C, the main portion of the beam assumingrthe shape of a cone with dimension bfas base., and the burner cavity having a surface of revolution.y Y The initial radiation from the burner cavity is distributed in all directions, i. e., almost 21r steradians inthe case illustrated; however, the intensity of radiation 1s not uniform, being greatest along or close to the axis of the cavity, which is shown to be perpendicular to the surface of the. wall A, and diminishing progressively with increasing angle farther from the said axis in a manner v'that depends upon the design of the burner, the shape of the cavityv being an important factor. The main part of the radiation is the central part, throughout all of which the radiation intensity is at least 75% of the maximum radiation intensity expressed, for example, iny B. t, u. per steradian per hour.

rThis main part is indicated in the drawing as a cone h aving an apex angle Of which has a magnitude determined by the design of the burner, e. g. about 70 to 100, these limits beingV merely illustrative. Beyond the main part the radiation intensity falls olf rapidly, and the radiation is, therefore, essentially a confined beam. In accordance with the invention the tubes are situated outside of the said main part of the radiation; although they may be placed just outside, it is preferred to place them at some distance beyond the main part, as shown, so that the intensity of radiations falling on the tube is well below of the maximum, e. g., 50% of the said maximum.

To achieve the intended purpose most efliciently it is preferable that the following three points should be observed:

l. To avoid as much as practically possible any spots on the wall C, which are not under the influence of direct radiation from the source of heat, the distance a between the burners should be less or equal to the base b of the radiating cone. (agb) 2. To avoid the impingement of the main portion of the direct radiation on the tubes, but to concentrate as many as possible once and twice reflected (re-radiated) rays on the total tube surface, the tubes must be placed outside these main portions of the radiating beams, and the shorter distance c between the centers of the tubes should be equal to or more than twice the outside tube diameter d.

3. For the same reason, as mentioned under 2, the distance e1 of the tubes from the wall A should preferably be less than one-half and more than one-eighth the distance e between both walls A and C.

Asl was indicated above, separate initial sources of radiation can be located on both walls; in this case both initial radiations are likewise confined or shielded from the tubes and both are repeatedly re-radiated between radiant walls, whereby they individually heat both sides of the tubes.

When the process fluid is iiowed through more than one tube Vit may be flowed either in several parallel streams or serially through two or more tubes, depending upon the flow velocity and residence time desired, and I do not wishv to limit myself to any specific tlow arrangement. The tubes, regardless of the flow circuit, should be spaced apart so as to permit radiation from one wall to another through the spaces between the tubes. The tubes may be arranged in one or in a plurality of rows. In general, adequate radiation between walls is best realized by limiting the number of tubes so that the sum of the projected areas of the tubes is not greater than half of the area of one wall. By projected area l mean the projection of the tube perpendicularly on the wall. In the case of annular radiant sections, wherein the concentric radiant walls are of unequal sizes, I refer to. the wall! fromwhich the second reradiation is emitted. When more than one row of tubes is provided the tubes should lbe arranged to avoid shielding of any tubes from either wall. The expression row of tubes is used to denote tubes atabout the same distance from a wall. Shielding is best avoided by locating the tubes so that everyV tube .is separated' from every other tube, whether in the same or in another row, by a clear interval equal to at least one ltube diameter.

To achieve most even distribution of heat absorption among the several tubes, I prefer to arrange the tubes' in small groups, such as groups of two as shown in Figs. l and 3 and, to provide a clear path between tubes within the same group at least equal to the. projected area of one tube, with greater intervals between groups, so as to make thev sum of the clear paths for all intervals. atleast twice the sumof; the projected areas ofthe several tubes. In this Vfn'eferred arrangement there is a source of radiant heat .between each pair ofv groups, opposite the greater intervals. When using the arrangement according to Figs.

graisse l and 3 heat is absorbed most evenly by mounting all tubes in a single row and providing two tubes within each group; however, this limitation is not in every case essential, and l may use more than two tubes within each group and use more than one row to economize on space.

According to another feature of the invention, I arrange the source of initial radiation linearly, parallel to the tube or tubes. Such a linear source of radiation may be continuous or discontinuous. Thus, ideally, a continuous line source, comprising an infinite number of point sources arranged in a row, is contemplated; the practical embodiment of the invention can be achieved by the use of radiant type burners with trough-like cavities extending parallel to the tubes for the continuous sources, or by the use of cup type radiant burners arranged in a row with finite intervals between centers, thereby producing a discontinuous linear source. By providing a linear source parallel to the tube or tubes, whether continuous or discontinuous, the tube or tubes are heated along their lengths. By this expedient it is possible to provide a uniform rate of heat absorption along the lengths of the tubes, or to provide any desired gradient by varying the spacing of the radiant type burners along the row, or by operating different parts of the trough-like burner or individual burners of the same or of different rows at dilerent rates of heat emission. It also provides for controlling the desired rate of heat absorption along the entire path of the process fluid when all or some tubes are connected in series, by arranging the manifolding through which fuel is supplied to the burners in such a manner as to operate individually each linear, vertical row of burners from a common fuel header controlled by a single valve.

The method of heating and the heater according to the invention make use of the fact that radiant heat can be advantageously employed for heating tubes if the generation of heat is well controlled and the heat-radiating surfaces are disposed for immission and emission of the radiated heat. The tubes are heated predominantly by radiation as the burners are usually of the radiant type and are operated with practically no excess combustion air. The space between the radiant walls assumes a temperature near that of the exit ue gas almost concurrently with combustion; hence control is readily elected and a rapid response to changes in the rate of heat emission is realized. Moreover, this circumstance makes it possible to control the rate of heat absorption at any point along the length of the tube or along the entire path of the process fluid and, hence, the equilibrium temperature.

In still another aspect, the present invention contemplates a composite heater comprising a radiant heating section, constructed as described above, and arranged as an annulus about a central preheating section, the wall of the latter forming also the inner wall of the radiant section. Combustiongases from the radiant type burners are withdrawn from the bottom of the annular radiant section and passed through a perforated wall construction, such as an open checkerbrick, into the bottom of the preheating section wherein they ascend and heat preheating tubes predominantly by convection. The preheat temperature is regulated by augmenting the ascending combustion gases with fresh, hot combustion gases from an auxiliary burner at the bottom of the preheating section. The latter should preferably have a high turn down range and a high heat capacity and be of the type operating with excess air so as to afford control of the total heat available in the preheating section over a Wide range.

The two walls defining the annular radiant section according to the foregoing paragraph may have any desired configuration, e. g., they may be parallel walls, having concentric, circular cross sections; or both may be polygonal; or either one may be polygonal and the other of circular cross section, as shown in Fig. 3. Walls of circular cross section will usually be employed when selfsupported walls are used; when extremely high temperatures are encountered and ceramic refractory and insulating walls suspended from external steel frameworks are necessary polygonal walls are more readily installed.

Referring to Figs. 2 to 5, the heater is shown to be supported on a concrete foundation 1 through structural steel framework 2. An external steel framework, indicated generally at 3, provides a rigid support for the outer Wall which is polygonal and comprises refractory flre bricks 4 and ceramic insulating blocks 5, suspended by lugs, not shown. An outer layer 6 of casing cement is applied to the insulating bricks. This Wall construction, apart from the arrangement of the radiant type burners, is well known and referred to as a Suspended Wall Construction. The floor comprises refractory lire bricks 7 resting on ceramic insulating bricks 8, supported by heat resistant concrete 9. The roof comprises tile bricks 10, suspended from the steel framework.

The inner cylindrical Wall comprises a perforated lower section 11, e. g., open checkerbrick, supporting a solid cylindrical section 12 of high-temperature resistant cast aggregate. It terminates beneath the roof so as to leave a slight clearance, permitting flow of a minor part of combustion gas from the top of the annular radiant section (between the two walls) into the top of the preheating section (within the inner wall). The ow channel over the top of the wall 12 is considerably smaller than that through the perforated section 11. A steel cylinder 13, closed on the bottom and covered with heat-resistant cement, is suspended from the steel framework and occupies the upper portion of the preheating section to reduce the cross sectional area to an annular space. Vertical slots 14 near the top permit the outflow of line gas into a stack 15.

A plurality of preheating tubes 16, usually one or two per each reaction tube, are arranged in a circle in the preheating section and can be provided with vertical fins 16a near the top portions thereof; they are supplied with fresh process fluid from an external manifold at the top, not shown. These tubes are connected to vertical main reaction tubes 17 at the bottom by one or more short transfer lines 18. 'Ihe reaction tubes are connected at the top to an external discharge manifold, not shown. Thermocouple wells 18a are provided in the transfer lines.

Each inner face of the octagonal outer wall is provided with a vertical row of radiant type burners 19, there being six burners in each row in the embodiment shown. This provides a complex of eight substantially linear sources of radiant heat. The burners are preferably of the type which effect substantially complete combustion within the disk-shaped burner cavity 20 without forming a projecting llame, and to distribute evenly the radiation resulting from the incandescence of the burner cavity. The main part of the radiation is conned within a conical beam having the apex angle 1, as shown in Fig. 3. Such burners preferably operate with a minimum of excess air, e. g., with stoichiometric fuel-air combination mixtures to provide for maximum radiant efficiency. It is desirable that the burners be adapted to be operated over a wide range of fuel flows, i. e., they have a high turn down range.

According to one specific arrangement shown in Fig. 4, each burner is mounted in a separate burner block Z1. The burner assembly is removably attached to the steel framework 3 by means of bolts, not shown. The burner plate or frame 22 includes a tubular portion forming a venturi constriction 22a communicating with an annular ceramic tip holder 23 into which a ceramic tip 24 is threaded. The threads on the tip have longitudinal slots 24a, extending also along the rear side of the forward part of the tip to provide narrow passages for the discharge of combustible gas tangentially to the outer face of the burner body 25. Gaseous fuel of any type, `premixed with a portion of the combustion air, is discharged through a nozzle 26 into the venturi throat 22a, thereby drawing in additional combustion air. The burner is initially ignited through a lighting hole 27, provided with Y 'I a Yhinged .cover 28. The refractory extension ring 29 has. a -low'heat conductivity, which assists in keeping incomingV fuel suiciently cool .to prevent pre-ignition without theV use of cooling water. Combustion takes place within theV cavity 20 immediately after issuance of the fuel-air mixture from the rear of the tip 24.

While I have described one specific type of radiant burner suitable for use with a heater constructed according to this invention, it should be understood that I may employ any other source of radiant heat which will produce radiation at a high temperature level within a relatively confined beam, e. g., one wherein oil is burned instead of gas, or burners wherein the cavity 20 is a vertical trough ina Yvertically elongated refractory body, and that I may employ a different number of burners in each row, so as to achieve a more nearly continuous line source of heat. The particular burner described is not my invention. i The group of burners within any vertical row-may be. regarded, collectively, as predominantly emitting radiant heat within a confined, more or less wedge-shaped zone diverging toward the inner wall, the limits of the wedge being also indicated by the angle fr, in Figure 3. It will be noted that the main reaction tubes 17 are located Voutside of these wedge-shaped zones, whereby direct radiation by the high intensity part of the radiant heat is avoided. YThe areas of the spaces between the tubes within each group of two are, in this embodiment, equal ,to about one and a half times the area of one tube, both areas being projected areas; a closer spacing may, of course, be employed. The tubes are located closer to the outer wall to avoid being exposed to the main part of the direct radiation.

As shown in Fig. 5, the burners are supplied with fuel from a gas line 30 which may, for example, carry natural gas. The gas is mixed with a portion, e. g., from 25 to v40% of the total combustion air, in a gas mixing device 31 and compressed in a compressor 32. This gas-air mixture, e. g., at about lbs. per sq. in; gauge, is passed into horizontal, annular header 34 from which it is passed through automatic fire checks 33 and distributedto the vertical headers 34a and to the individual burners through valves 35 and 35a. the rate of fuel flow to an entire vertical row of burners. The component parts of the supply system described in Ythis paragraph are known and not parts of this invention; further, the invention is notV restricted to the use of the supply system described. An auxiliary burner 36 is mounted at the bottom of vthe preheating section. This is preferably equipped by acylindricalV muilie 37 to prevent direct impingement of the llame on the preheating tubes 16. The burner 36 has. a control valve 38 and is preferably of the type having a short flame characteristic and a high turn down range.

Inoperating the heater, process uid is admitted to the Vtops of the preheat tubes 16 and heated predominantly by convection by combustion gases from the radiant.

burners '19 entering the bottom of the preheating section through' the perforated section 11 and by the combustion gas of therauxiliary burner 36. Flue gas is discharged through the slots 14 and stack 15. A minor amount of the. combustion gas from the radiant burners passes over the top of the wall 12, but the main flow of these gases is downward within the annular radiant section for the purpose of utilizing the heat of these combustion gases in the preheating section. The temperature to which the process fluid is preheated can be regulated by the operation of the auxiliary burner 36.

The vaporized process iluid which was preheated to reaction temperature moves upwardly' through the reaction tubes 1'7, which are heated by radiant and reradiated heat. Only the low-intensity part of the radiant heat emitted from the burners 19 'strikes the tubes, and

Valves 35 control most of this heat is immtted on and partly re-radiated by theinner wall 12 tothe tubes 17 and the outer wall 4. From the latter itis again partly re-radiated onto the sides of the tubesY 17 facing toward the wall 4, and onto the inner wall 12. From the wall 12 further re-radiation takes place, the heat being thus repeatedly re-radiated until its intensity diminishes to a level at which it can bevneglected. The intensity of heating along the lengths of the tubes can be controlled by manipulating the valves a, while the intensity of heating of an entireV tube as a whole can be controlled by manipulating the corresponding valve 35.

The fraction of the total heat release that is radiated from the burner cavities depends upon the completeness of the combustion, and it is preferred to employ burners wherein this is achieved to a high degree. Withsuch burners the width of the layer of gas in the annular radiant section is small compared with the conventional type radiant heaters; this will lessen the heat absorptivity and radiation effect of the yintervening lgas layers and tends toward more even distribution of heat absorption. The downward velocity of the combustion gas in this section is relatively low, so that heating by convection plays a small part in the heating of the tubes.

While I have shown a heaterV wherein the combustion products from the radiant burners 19 are, for the most part, discharged at the bottom through the perforated Y section 11, I may provide exit passages through the inner wall 12 at any suitable level, or at a plurality of levels, thereby further reducing heating by convection.

The invention may be embodied in other ways. One such alternate embodiment is shown vin Figs. 6 and 7 wherein the heater comprises opposed radiant walls 40 and 41, end walls 42 and 43, a oor 44 and a roof 45, all made of ceramic refractory and insulating material. Horizontal reaction tubes 46 and 47 are located in two vertical tiers or rows, each row being parallel to the walls with alternating large and small intervals in each row. The tubes are staggered to provide a zig-zag pattern,

readily recognized in Fig. 6. These tubes are spaced from the walls 40 and 41, preferably by a distance not less than one fourth of the distance between these walls.

Each tube is spaced from the tube nearest it, regardless of tier, by yat least one tube diameter. The tubes may be connected externally either in series or parallel or series-parallel by headers, not shown. Each radiant wall is provided with horizontal rows of radiant burners 48, located to emit radiant heat as beams the main parts of which do not intercept, the tubes. The rows of burners on opposite walls are staggered as shown, each row being .s located at a level opposite a smaller interval in thel nearest tier of tubes. Combustion gas is discharged from the bottom through a checkerbrick 49 affording aue passage into a stack 50 or into a preheating section, as previously described.

In this embodiment process fluid is owed through the horizontal tubes along any desired circuit. Each row of burners heats the tubes on both sides by repeated re-radiation in the manner previously described forfFigs. 2-5, i. e., each source of initial radiation operates to heat both sides of the tubes. This embodiment effects some saving in space in that more tubes can be accommodated for a given length of wall measured transversely to the tube axes.

While l have, for purposes of illustration, shown a Y heaters on both walls can evidently be applied to installations employing vertical tubes as previously described and illustrated; Figs. 6 and 7 serve to illustrate also such an arrangement, the walls 40, 4i, 44 and 45 being in this instance all vertical.

This application is a continuation-impart of application Ser. No. 19,243, tiled April 6, 1948, and now abandoned.

I claim as my invention:

l. The method of transferring heat to a fluid comprising: flowing said duid through a tube located between two radiant walls; radiating heat from an initial source near the first of said walls essentially as a confined beam predominantly against a limited area of the facing surface of the second wall with only a small portion of the radiated energy falling directly on said tube, the main part of said beam, containing all high intensity direct radiations, being unobstructed; re-radiating heat from the same surface of said second wall; absorbing part of said re-radiated heat on the side of the tube exposed toward said second wall and immitting another part of the reradiated heat on the facing surface of the rst wall; re-radiating immitted heat from the same surface of said first wall; and absorbing a portion of the heat re-radiated from said irst wall on the side of the tube exposed toward said first wall, the said small portion of the radiated energy falling directly on said tube being a minor part of the total radiated heat absorbed by the tube.

2. The method of transferring heat to a fluid comprising: flowing said fluid through a tube located between first and second radiant walls; maintaining an incandescent source of radiant heat near said rst wall by burning fuel thereat substantially completely within an open cavity; emitting radiant heat from said incandescent source essentially as a confined beam predominantly against a limited area of the facing surface of the second wall with only a small portion of the radiated energy falling directly on said tube, the main part of said beam, wherein the radiation intensity is at least 75% of the maximum radiation intensity of the beam, being unobstructed; re-radiating heat from the same surface of said second wall; absorbing part of said re-radiated heat on the side of the tube exposed toward said second wall and immitting another part of the re-radiated heat on the facing surface of the first wall; re-radiating immitted heat from the same surface of the rst wall; and absorbing a portion of the heat re-radiated from said rst wall on the side of the tube exposed toward said first wall, the said small portion of the radiated energy falling directly on said tube being less than 25% of the total radiated energy absorbed by the tube.

3. The method of transferring heat to a duid comprising: flowing said uid through an elongated tube located between rst and second radiant walls; maintaining a substantially linear source of radiant heat near said rst wall, said substantially linear source being substantially parallel to said tube and oset laterally from the said tube; radiating heat from said source essentially as a confined elongated beam predominanetly against a limited area of the facing surface of the second wall elongated in a direction parallel to the tube, said beam being directed so that the main part thereof, including all direct radiation from said source having an intensity of at least 75% of the maximum radiation intensity being unobstructed; re-radiating heat from the same surface of said second wall; absorbing part of said re-radiated heat on the side of the tube exposed toward said second wall and immitting another part of the re-radiated heat on the facing surface of the first wall; re-radiating immitted heat from the same surface of the first wall; and absorbing a portion of the heat re-radiated from said rst wall on the side of the tube exposed toward said tirst wall.

4. The method according to claim 3, wherein the substantially linear source of heat s maintained by burning fuel within a plurality of open cavities disposed in a row at said first wall, thereby providing a plurality of incan'A descent points.

5. The method according to claim 3, wherein an additional, substantially linear source of radiant heat is simultaneously maintained near said second wall substantially parallel to said tube and offset laterally therefrom; heat being radiated from said additional linear source essentially as an additional confined beam predominantly against a limited area of the facing surface of the rst wall elongated in a direction parallel to the tube, said additional beam being directed so that the main part thereof, including all direct radiations of intensity in excess of 75 of the maximum radiation intensity, is unobstructed; heat immitted by said additional beam being re-radiated from the same surface of said first Wall and then partly absorbed by the side of the tube exposed toward said first wall and partly immitted on the facing surface of said second wall; and the last-mentioned immitted heat being re-radiated from the same surface of said second Wall and then partly absorbed by the side of the tube exposed toward said second wall.

6. A radiant heater including: a radiant heating chamber having opposed radiant walls; a narrow radiant heat acceptor located between and spaced from said walls and having a projected area less than the area of one wall and the space to each side of the acceptor in directions parallel to the walls being unobstructed to permit radiation between said walls; and a source of radiant heat near said one wall adapted to emit radiant heat essentially as a confined beam from said wall predominantly against a limited area of the other wall to heat said radiant heat acceptor by re-radiation from said other wall, the space between said walls traversed by the main part of said beam, containing all high-intensity direct radiations, being unobstructed.

7. A radiant heater including: a radiant heating chamber having opposed radiant walls; a plurality of radiantly heated tubes located between and spaced from said walls, the sum of the projected areas of all said tubes being less than the area of one wall and each tube having a clear space on each side thereof in a direction parallel to the walls to permit radiation between said walls; and a source of radiant heat near said one wall adapted to emit radiant heat essentially as a confined beam predominantly against a limited area of the other wall to heat said tubes by re-radiation from said walls, all said tubes being located out of the main part of said beam, said main part including all direct radiations from said source having an intensity of at least 75 of the maximum radiation intensity.

8. The radiant heater according to claim 7 wherein the tubes are spaced apart so that the distance between the center of any tube and that of the tube nearest thereto is at least twice the diameter of the tube.

9. The radiant heater according to claim 7 wherein the sum of the projected areas of the tubes is-not more than half of the area of said one Wall.

10. The radiant heater according to claim 9 wherein the tubes are arranged in a row located at a distance from said one wall more than one-eighth and less than onehalf of the distance between said walls.

t 1l. A radiant heater including: a radiant heating chamber having opposed radiant walls; a plurality of radiantly heated tubes located in at least one row between said walls and spaced therefrom, the sum of the projected areas of all the intervals between tubes being at least twice the sum of the projected areas of all the tubes, said tubes being arranged within each row in groups with relatively larger intervals between groups and relatively smaller intervals between tubes within each group to permit radiation between said walls; and a plurality of sources of radiant heat near one of said walls adapted to emit radiant heat essentially as confined beams predominantly against limited areas of the other Wall to heat said tubes by re-radiation from said walls, said tubes being frei-,sse

Y l l situated toV be out ofv the main, high intensity parts of said beamsV and to cause said main parts ofV thejbeams to passvthrough said' larger intervals against the said other wall.V

12. The radiant heater according to claim 1l wherein the sources of radiant heat are disposed so that the said main parts of the beams pass through adjacent intervals between groups of tubes and intersect after passage through said intervals and before immission on the opposite wall, whereby the said limited. areas of the other Wall overlap.

13. A radiant heater including: a radiant heating chamber having opposedv radiant walls; a plurality of radiantly heated tubes located in a plurality oi'rows between and substantially parallel to said walls and spaced therefrom, said tubes having a total projected area less than the area ofV one wall, said tubes being arranged Within each row with alternating relatively larger and smaller intervals and the rows being staggered to produce a zig-zag pattern to permit radiation between said walls; and a plurality of sources of radiant heat near said one wall adapted to emit radiant heat essentially as conrined beams predominantly against limited areas of the other wall to heat said tubes by re-radiation from said other wall, saidtubes being situated to be out of the main, high intensity parts of said beams and to cause said main parts of the beams to pass through the relatively smaller intervals of the row nearest said one Wall and thence through the relatively larger intervals in the row nearest said other wall.

14. In combination with the radiant heater according to claim 13, a plurality of sources of radiant heat on said other wall adapted to emit radiant heat essentially as confined beams, the main, high intensity parts of which pass through the relatively smaller intervals of the row nearest said other wall and thence through the relatively larger intervals Vin the row nearest said one predominantly against limited areas on said one wall without impinging on said tubes to heat said tubes by re-radiation from said one wall.

15. A radiant heater including: a radiant heating chamber having opposed radiant walls; a plurality of radiantly heated tubes located between and spaced from sai-d walls and from each other, the sum of the projected areas of all saidtubes being less than the area of one of said walls to permit radiation between said walls; and a complex of substantially linear sources of radiant heat near said one wall extending substantially'parallel to said tubes andopposite the intervals between the tubes, said sources being adapted to emit radiantheat essentially as coniined beams elongated in a direction parallel to said tubes predominantly against limited areas of the other wall to heat said tubes by re-radiation from said walls, all said tubes being situated to be out of the main, high intensity parts of said beams.

16; The radiant heater according to claim 15 wherein the tubes are arranged in a single row in groups of two, the distance between centers of the tubes within each group being at least twice the diameter of the tubes and the clear intervals between groups being greater than the interval between tubes within the same group; wherein said row is located at a distance from said one wall more than about one-eighth and less than one-half of the distance between said walls; and wherein the linear sources of heat are arranged to emit radiant heat against said other wall through said intervals between groups of tubes.

' 17. The radiant heater according to claim 15 wherein a second complex ot linear sources of radiant heat is provided near said other wall,l said second sources being adapted toemit radiant heat essentially as additional con-v fined beams elongated in a direction parallel to said tubes predominantly against limited areas of the said one Wall to further heat said tubes by re-radiation from said walls, all said tubes being out of the main, high intensity parts of said additional beams.

I8, A radiant tubular heater including: a pair of con- 12 centric radiant Walls forming an annular radiant heating section between them, the inner wall deiining within itself a convection preheating section whollywithin said radiant section; radi'antly heated conversion tubes within said radiant section; preheating tubes within'the preheating section connected by transfer lines to said radiantly heated conversion tubes; burners in sai-d radiant section to heat said conversion tubes by radiation; a passageway for combustion gas connecting the annular radiant section to the preheating section for the iiow of combustion gas from the radiant section into the preheating section; and i ue means in communication with the preheating section at a point spaced from said passageway for discharge of said combustion gas after traversing at least a part of the preheating section. I

19. En combination Ywith the tubular heater according to claim 1S, an auxiliary source of hot combustion gases for said preheating section.

20. A radiant tubular heater including: a pair of concentric radiant walls forming an annular radiant heating section between them, the inner wall defining within itself` a convection preheating section wholly within said radiant section; radiantly heated conversion tubes within said radiant section; preheating tubes withinV the preheating section connected by short radial transfer lines to said radiantly heated conversion tubes; burners in said radiant section to heat said conversion tubes by radiation; va passageway for combustion gas connecting the lower part of the annular radiant section to the lower part of the preheating section for the flow of combustion gas'from the radiant section into the preheating section; and flue means for the discharge of combustion gas from the top of the preheating section.

2l. A radiant tubular heater including: a pair of con'- centric radiant walls forming an annular radiant heating section between them, the inner wall deiining within itself a convection preheating section wholly within said radiant section; vertical radiant'ly heated conversion tubes within said radiant section; preheating tubes within the. preheating section connected by transfer lines to said radiantly heated conversion tubes; a plurality of vertical rows of radiant burners in said radiant section for heating said conversion tubes by radiation; a passageway aording a circumferentially distributed communication between the lower part of the annular radiant section and the preheating section for the ow of combustion gas yfrom the radiant section into the preheating section; an auxiliary burner in the lower part of said preheating section for supplying additional hot combustion gases; and flue means for the discharge of combustion gas from the top of the preheating section.

22. A radiant heater including: a pair ot' concentric radiant walls forming an annular radiant heating section between them; a plurality of Vertical radiantly heated tubes located within said heating section and spaced from said walls, the sum of the projected areas of said tubes on the outer of said walls being not more than half of the area of the said outer wall and said tubes beingk spaced apartto permit radiation between said walls; Vand a compleX of substantially linear, vertically elongated sources of radiant heat Vnear one of said walls extending substantially parallel to said tubes and opposite the intervals between said tubes, said sources being adapted to radiate heat essentially as confined beams predominantly against limited areas of the other wall to heat saidtubes by reradiation from said walls, said tubes being situated to be out of. theV main, high intensity parts of said beams.

2.3. A radiant `heater including: a pair of concentric radiant walls forming an annular radiant heating charnberl between them; a plurality of vertical radiantly .heated tubes located in at least Vone row between said walls and spaced therefronnthe sum of the projected areas of said tubes being aminor fraction of the area, of the outer wall, said tubes being spaced apart and arranged in groups with relatively largerfintervals between groups and relatively smaller intervals between tubes within each group to permit radiation between said walls; and a complex of substantially linear sources of radiant heat near said outer wall opposite a plurality of said larger intervals adapted to emit radiant heat essentially as conned, vertically elongated beams predominantly against limited areas or the inner wall to heat said tubes by re-radiation from said walls, said tubes being situated out of the main parts of said beams, said main parts including all direct radiations from the respective sources having intensities of at least 75% of the maximum radiation intensities.

24. A radiant tubular heater including: a pair of concentric radiant walls forming an annular radiant heating section between them, the inner wall defining within itself a central preheating section; a plurality of preheating tubes within the preheating section; a plurality of radiantly heated tubes connected to said preheating tubes and located vertically within the annular radiant section and spaced from said walls, the sum of the projected areas of said radiantly heated tubes constituting a minor fraction of the area of the outer wall and said radiantly heated tubes being spaced apart to permit radiation between said walls; a complex of radiant burners arranged to form linear sources of radiant heat near said outer wall adapted to emit radiant heat essentially as conned, vertically elongated beams predominantly against limited areas of the inner wall to heat said tubes by re-radiation from said walls, said radiantly heated tubes being situated outside of the main parts of the beams that include all direct radiations from the respective sources with intensities of at least 75% of the maximum radiation intensities; a passageway for combustion gas between the lower part of the annular radiant section and the lower part of the preheating section; and iue means for the discharge of combustion gas from the top of the preheating section.

25. In combination with the tubular heater according to claim 24, an auxiliary source of hot combustion gases for said preheating section.

26. A radiant tubular converter including: a pair of concentric radiant walls forming an annular radiant section between them, the inner wall defining within itself a central preheating section; a communicating passageway connecting the lower portions of said radiant section and said preheating section; a plurality of preheating tubes extending vertically within said preheating section; a plurality of radiantly heated conversion tubes within the an- 14 nular radiant section spaced from said walls, the sum of the projected areas of said conversion tubes being not more than one-third of the area of the outer wall, said conversion tubes being spaced apart and arranged in groups with relatively larger intervals between groups and relatively smaller intervals between tubes within each group to permit radiation between said walls; a pluralityA of vertical rows of radiant burners at the outer wall opposite a corresponding plurality of said larger intervals, each of said burners having a concave focusing burner cavity adapted to radiate heat essentially as a confined beam predominantly against a limited area of the inner wall to heat said tubes by re-radiation from said walls, said burners being of the type wherein substantially complete combustion is achieved within the burner cavity, said conversion tubes being located out of the main, high intensity parts of said beams and being in ow communication with said preheating tubes; an auxiliary burner at the bottom of said preheating Section; and flue means for the discharge of combustion gas from the top of the preheating section.

References Cited in the tile of this patent UNITED STATES PATENTS 1,523,657 Mather Jan. 20, 1925 1,747,421 Burroughs Feb. 18, 1930 2,002,463 Bailey et al. May 21, 1935 2,029,292 Alther Feb. 4, 1936 2,029,293 Alther Feb. 4, 1936 2,081,927 Hassler et al. June 1, 1937 2,087,800 Bailey et al July 20, 1937 2,101,835 Alcorn Dec. 14, 1937 2,114,269 Moore Apr. 12, 1938 2,215,079 Hess Sept. 17, 1940 2,215,080 Hess Sept. 17, 1940 2,215,081 Hess Sept. 17, 1940 2,219,860 Zimmerman Oct. 29, 1940 2,220,387 Baker Nov. 5, 1940 2,223,379 Melder Dec. 3, 1940 2,258,235 Barnes Oct. 7, 1941 2,277,595 Levy et al Mar. 24, 1942 2,288,366 Parsons .lune 30, 1942 2,306,818 Lyster Dec. 29, 1942 2,523,971 Schutt Sept. 26, 1950 2,527,410 Fleischer Oct. 24, 1950 

